The ‘Forbidden Planet’ has been found in the Neptunian Desert

Artist's impression of NGTS-4b transiting its parent star

An exoplanet smaller than Neptune with its own atmosphere has been discovered in the so-called ‘Neptunian Desert’ by an international collaboration of astronomers, including researchers from the University of Leicester.

New research by an international group of researchers, including Dr Matt Burleigh and Dr Emma Longstaff of the Department of Physics and Astronomy at the University of Leicester, has identified a rogue planet.

It was discovered by using the state-of-the-art Next-Generation Transit Survey (NGTS) facility, which is situated at the European Southern Observatory's Paranal Observatory in the heart of the Atacama Desert in Chile. It is a collaboration led by the Astronomy and Astrophysics Group at the University of Warwick, together with the University of Leicester, University of Cambridge and Queen's University Belfast in the UK, and the Observatoire de Genève, DLR Berlin and Universidad de Chile.

NGTS-4b, also nicknamed the ‘Forbidden Planet’ by researchers, is a planet 20% smaller than Neptune but three times the size of Earth. It has a mass of 20 Earth masses, and is hotter than Mercury at 1,000 degrees Celsius. It orbits around its parent star in only 1.3 days.

It is the first exoplanet of its kind to have been found in the ‘Neptunian Desert’, the region close to stars where no Neptune-sized planets are normally found. This area receives strong irradiation from its star, meaning the planets do not retain their gaseous atmosphere – they evaporate, leaving just a rocky core. Notably, NGTS-4b still has its atmosphere of gas.

When looking for new planets, astronomers look for a dip in the light of a star. This shows that the planet is in transit - orbiting it and blocking the light. Usually only dips of 1% and more are picked up by ground-based searches, but the NGTS telescopes can pick up a dip of just 0.2%, which was essential to the detection of NGTS-4b.

This discovery hinged on a crucial observation taken by University of Leicester astronomers, using a telescope at the South African Astronomical Observatory: Dr Burleigh and Dr Longstaff confirmed that the transit, and therefore the exoplanet, was real. This alerted the rest of the NGTS consortium to the exciting potential of this exoplanet, and prompted a rush to collect further data.
Researchers believe the planet may have moved into the Neptunian Desert recently, in the last one million years, or that it was very big and its atmosphere is still evaporating.

Dr Burleigh said: “At Leicester we have been leading a campaign to follow-up and confirm NGTS’s exoplanet discoveries. Since this transit is so shallow, NGTS-4b wasn’t initially one of our top priority targets. But thanks to the excellent facilities in South Africa, we were able to detect the transit and convince ourselves the planet was real.”

Dr Richard West, from the Department of Physics at the University of Warwick, commented: “This planet must be tough - it is right in the zone where we expected Neptune-sized planets could not survive. It is truly remarkable that we found a transiting planet via a star dimming by less than 0.2% - this has never been done before by telescopes on the ground, and it was great to find after working on this project for a year.

“We are now scouring out data to see if we can see any more planets in the Neptune Desert – perhaps the desert is greener than was once thought.”